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A radio clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock. Such a clock may be synchronized to the time sent by a single transmitter, such as many national or regional time transmitters, or may use multiple transmitters, like the Global Positioning System. Such systems may be used to set computer clocks or clocks meant for human readability.

Single transmitter

Radio clocks synchronized to terrestrial time signals can usually achieve an accuracy of around 1 millisecond relative to the time standard (citation ?), generally limited by uncertainties and variability in radio propagation.

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Longwave and shortwave transmissions

Radio clocks depend on time signal of radio stations. These time standards specify:

the broadcast frequency of the frequency standard

the exact geographic location of each antenna, so the radio signal’s time of propagation can be estimated

Many other countries can receive these signals (JJY can sometimes be received in Western Australia, Tasmania, and the Pacific Northwest of North America at night), but it depends on the time of day, atmospheric conditions, and interference from intervening buildings. Reception is generally better if the clock is placed near a window facing the transmitter. There is also a transit delay of approximately 1 ms for every 300 km the receiver is from the transmitter. When operating properly and correctly synchronized, better brands of radio clocks are normally accurate to the second. (Product advertising often claims higher accuracy, but for many or most users that is only a theoretical possibility.)[citation needed]

Clock receivers

Many manufacturers and retailers sell radio clocks under the name "atomic clocks", but the clocks themselves are not atomic. Instead, they receive coded time signals from a radio station, which, in turn, derives the time from a true atomic clock.

One of the first radio clocks was offered by Heathkit in late 1983. Their model GC-1000 "Most Accurate Clock" received shortwave time signals from radio station WWV in Colorado whenever propagation conditions permitted, automatically switching between the 5, 10, and 15 MHz frequencies to find the strongest signal as conditions changed through the day and year. It kept time during periods of poor reception with a quartz-crystal oscillator. This oscillator was disciplined, meaning that the microprocessor-based clock used the highly accurate frequency standard signal received from WWV to trim the crystal oscillator. The timekeeping between updates was thus considerably more accurate than the crystal alone could have achieved. Time down to the tenth of a second was shown on an LED display. The GC-1000 originally sold for $250 in kit form, $400 preassembled, and was considered impressive at the time. Heath Company was granted a patent for their design. [1][2]

In the 2000s, radio-based "atomic clocks" became common in retail stores. Simple units can be purchased in the United States at most electronics or discount stores for $20 to $50 and often feature wireless outdoor and indoor thermometers. These use the longwave signal from WWVB. They require placement in a location with a relatively unobstructed atmospheric path to the transmitter, perform synchronization only once a day during the nighttime, and need fair to good atmospheric conditions to successfully update the time. The device that keeps track of the time between updates, or in their absence, is usually a relatively inaccurate non-disciplined quartz-crystal clock, since it is thought that an expensive precise time keeper is not necessary with automatic atomic clock updates. The clock may include an indicator to alert users to possible inaccuracy when synchronization has not been successful within the last 24 to 48 hours. In other cases, the indicator will indicate that synchronization has been achieved within the last few hours, and will go blank in the mid-morning.

Modern radio clocks can be referenced to atomic clocks, and provide a means of accessing high-quality atomic-derived time over a wide area using inexpensive equipment. However, radio clocks are not appropriate for high-precision scientific work.

Other broadcasts

Many analog broadcast stations also transmit a distinctive tone or tones at the precise top of every hour, derived from an official source. Most well known is the Greenwich Time Signal, transmitted on BBC radio since 1924. In the US, WTIC in Hartford, Connecticut has broadcast the Morse code letter "V" every hour, on the hour, since 1943.

Attached to other broadcast stations

Broadcast stations in many countries have carriers precisely synchronized to a standard phase and frequency, such as the BBC Radio 4longwave service on 198 kHz, and some also transmit sub-audible time-code information, like the Radio France longwave transmitter on 162 kHz. Many digital radio and digital television schemes also include provisions for time-code transmission.

DRM is able to send a clock signal, but one not as precise as GPS-Glonass clock signals.

Mobile telephones

Some mobile telephone technologies, such as Qualcomm's CDMA, are designed to distribute high-quality standard time signals (referenced to GPS in the case of CDMA). CDMA clocks are increasingly popular for providing reference time to computer networks. Their precision is nearly as good as that of GPS clocks, but since the signal comes from a nearby cell phone base station rather than a distant satellite, CDMA clocks generally work better inside buildings. So in many cases, when a GPS reference clock would require installing an outdoor antenna, a CDMA clock can overcome this requirement.

Multiple transmitters

Multiple time sources may be combined to derive a more accurate time synchronization sources. This is what is done in satellite navigation systems such as the Global Positioning System. GPS, Galileo and GLONASSsatellite navigation systems have a caesium or rubidium atomic clock on each satellite, referenced to a clock or clocks on the ground. Some navigation units can serve as local time standards, with a precision of about one microsecond (µs). The recent revival and enhancement of the terrestrial based radio navigation system, LORAN will provide another multiple source time distribution system.

GPS clocks

Many modern radio clocks use the Global Positioning System to provide more accurate time than can be obtained from these terrestrial radio stations. These GPS clocks combine time estimates from multiple satellite atomic clocks with error estimates maintained by a network of ground stations. Due to effects inherent in radio propagation and ionospheric spread and delay, GPS timing requires averaging over several periods of these phenomena. No GPS receiver directly computes time or frequency, rather they use GPS to discipline an oscillator that may range from a quartz crystal in a low-end navigation receiver, through oven-controlled crystal oscillators (OXCO) in specialized units, to atomic oscillators (Rubidium) in some receivers used for synchronization in telecommunications. For this reason, these devices are technically referred to as GPS-disciplined oscillators.

GPS units intended primarily for time measurement as opposed to navigation can be set to assume the antenna position is fixed; in this mode the device will average its position fixes so that after a day or so of operation it will know its position to within a few meters. Once it has averaged its position, it can then determine accurate time even if it can only pick up signals from one or two satellites. GPS clocks provide the precise time needed for Synchrophasor measurement of the electrical waves on an electricity grid to determine the health of the system.

Galileo positioning system

Using the Global Positioning System is dependent on the goodwill of the United States government for the operation of the GPS satellite constellation. This is not acceptable for many critical non-US civilian and military systems, although it may be acceptable for many civilian purposes, as it is assumed by most users that the civilian GPS signal would not be switched off except in the event of a global crisis of unprecedented proportions.

The planned establishment of the Galileo positioning system by the EU (expected to be fully operational in 2013) is intended to provide a second source of time for GPS-compatible clocks that are also equipped to receive and decode the Galileo signals.

LORAN

Renewed interest in LORAN applications and development has recently appeared as an augmentation to GPS and other GNSS systems. Enhanced LORAN, also known as eLORAN or E-LORAN, comprises an advancement in receiver design and transmission characteristics which increase the accuracy and usefulness of traditional LORAN to that comparable with unenhanced GPS. eLoran also includes additional pulses which can transmit auxiliary data such as DGPS corrections and UTC information. eLoran receivers now use "all in view" reception, incorporating signals from all stations in range.

Astronomy timekeeping

Although any GPS receiver that is performing its primary navigational function must have an internal time reference accurate to a small fraction of a second, the displayed time on most consumer GPS units may not be as precise. This is because an inexpensive GPS unit typically has one CPU that is multitasking; the highest-priority task for the CPU is maintaining satellite lock, while updating the display gets a lower priority. Therefore, the displayed time of most consumer handheld GPS units will be accurate to around half a second — more than sufficient accuracy for most civil timekeeping purposes, but not for scientific applications such as astronomy.

For serious precision timekeeping, a more specialized GPS device is needed. Some amateur astronomers, most notably those who time grazing lunar occultation events when the moon blocks the light from stars and planets, require the highest precision available for persons working outside large research institutions. The Web site of the International Occultation Timing Association has detailed technical information about precision timekeeping for the amateur astronomer.